The present invention relates to an atom probe and in particular to a scanning atom probe.
A known design of an atom probe is described in an article by A. Cerezo et al published in EMSA Bulletin 20:2 November 1990. The design essentially comprises a counter electrode and a detector located some distance (e.g. approx. 600 mm) behind the counter electrode. At least a part of the counter electrode is adapted (e.g. by having a hole formed therein) to permit the through-passage of ions. A sample to be analysed, in the form of a sharply pointed needle of end radius approximately 20-100 nm, is brought in front of the counter electrode so as to be substantially aligned with that part of the counter electrode which is adapted to permit the through-passage of ions. By applying a large positive voltage between the microtip and the counter electrode a sufficiently large electric field can be generated at the needle end to ionise exposed atoms thereon. The ions thus formed evaporate, leave the needle and are then accelerated towards the counter electrode. The ions which move towards that part of the counter electrode which is adapted to permit the through-passage of ions, continue beyond the electrode to the detector.
It is possible in theory to identify an ion removed from a sample using time-of-flight mass spectrometry. This requires knowledge of the ion""s time of flight, tf, which is typically a few microseconds, in addition to the ion""s kinetic energy. In order to know the time of flight accurately, it is necessary to know both when the ion left the needle and when it arrived at the detector with a high degree of precision. In order to determine accurately when the ions leave the needle end a very short positive voltage pulse Vp is generally applied to the sample on top of a positive dc biasing voltage, Vs, which is continuously applied to the sample. Alternatively, a short negative voltage pulse may be applied instead to the electrode. The dc voltage Vs is less than but near to that required to produce an electric field strength at the needle end which is sufficient to cause ionisation and evaporation of exposed or prominent atoms on the needle end. When the voltage pulse Vp is applied to the sample, the electric field strength is increased, at a well defined point in time for a well defined duration, such that ionisation and evaporation occurs. In this way the point in time at which an ion leaves the sample can be determined. However, the use of a voltage pulse gives rise to the situation that ions are accelerated away from the needle in a time-varying electric field which results in a broad spread in the kinetic energies of the ions travelling to the detector. Thus the kinetic energy of individual ions leaving the sample and travelling to the counter electrode is not well defined.
The present invention seeks to provide an atom probe which provides a narrow spread of kinetic energies of ions so as to provide improved mass resolution of detected ions. In a preferred embodiment the kinetic energy is substantially defined by the voltage potential between specimen and detector.
According to the present invention, there is provided an atom probe for analysing the surface of a specimen comprising a detector; a counter electrode which includes a first plate and at least one second plate located between the first plate and the detector; control and power supply means connected to the first and second plates for maintaining at least one second plate at a substantially constant voltage with respect to the detector and for supplying a negative voltage pulse to the first plate wherein the control and supply means is adapted to supply a voltage pulse which has a substantially constant or slowly varying maximum negative voltage for a period of time dependent on the separation of at least one second plate from the specimen.
Preferably, the first plate is maintained at substantially the same voltage as the at least one second plate other than when the negative voltage pulse is applied to the first plate when its voltage preferably falls below that of the at least one second plate.
Furthermore, ideally the at least one second plate and the detector are held at substantially the same voltage so that ions passing the first electrode are decelerated and then coast towards the detector.
Also, a preferred separation between the first and second plates is less than 250 xcexcm. This has the advantage of enabling the first plate to be located further than 1 xcexcm from the tip of the specimen to be analysed (preferably about 5-30 xcexcm) while still enabling ions leaving the specimen, when accelerated by a potential difference of approximately 10 kV, to reach the second plate within about 1 nanosecond. This is approximately the preferred accuracy within which the time of evaporation needs to be known in order to prevent significant loss of resolution.
The specimen may take the form of a sharply pointed needle of end radius approximately 20-100 nm or a substantially flat surface having one or more microtips thereon.
By maintaining the potential at the second plate substantially constant during the travel time of an ion between the needle end or microtip and the at least one second plate (or by varying the potential only relatively slowly during this time, typically less than 1 ns), the final kinetic energy of the ion will not depend strongly on the exact value of the voltage on the first plate at the time of evaporation. As a result, the spread of kinetic energies of ions accelerated in this way is relatively narrow. Any changes in the voltage of the first plate after an ion has passed the second plate will not affect the kinetic energy of the ion.
The control and supply means preferably incorporates at least one solid-state switch that provides the voltage pulse to the first plate. The use of one or more solid-state switches enables a voltage pulse to be provided which has a relatively smooth rise and fall and a plateau duration of approximately 1 ns. Such a voltage pulse is well adapted for use with the atom probe of the present invention.
The detector is preferably position sensitive such that a three dimensional analysis of the specimen may be undertaken.
Furthermore, the counter electrode is preferably mounted on drive means to enable the counter electrode to be moved relative to the specimen so as to permit alignment of the counter electrode with a sharply pointed specimen and/or to permit a number of different microtips on a substantially flat surface to be analysed sequentially. In the latter case, the atom probe is able to function as a scanning atom probe. Alternatively, the specimen may be moved with respect to a fixed/movable counter electrode to afford relative movement.
At least the first plate of the counter electrode may be generally cone-shaped in the direction away from the detector. This enables the electric field generated at the specimen to be concentrated at only a single microtip, thus preventing interference from any nearby microtips.
The counter electrode may conveniently be formed from two separately fabricated plates which are then assembled together with insulating spacers which provide insulation between the plates, ensure a suitable separation between the plates and assist in the positioning and relative alignment of the plates. Alternatively, the counter electrode may be integrally formed using a combination of metal and insulator layers on a shaped substrate.